Control of Prokaryotic Gene Expression - Operons

Question 1

Define: Operon

Answer 1

Cluster of genes with related functions; consisting of an operator, a common promoter and 2 or more structural genes, controlled as a single unit to produce a single polycistronic mRNA

Question 2

Negative control

Answer 2

Suggests that regulatory gene of operon encodes for a REPRESSOR

Question 3

Positive control

Answer 3

Suggests that regulatory gene encodes for an ACTIVATOR

Question 4

Inducible system

Answer 4

• Suggests that default state of operon is OFF, but can be turned on in the presence of an INDUCER

Question 5

Repressible system

Answer 5

• Suggests that default state of operon is ON, but can be turned off in the presence of a CO-REPRESSOR

Question 6

Negative inducible control

Answer 6

• repressor normally bound to operator, preventing binding of RNA polymerase to promoter; no transcription of structural genes
• presence of inducer inactivates repressor by altering tertiary structure and making it unable to bind to operator; transcription of structural genes

Question 7

Negative repressible control

Answer 7

• repressor unable to bind to operator in normal conformation, allowing RNA polymerase to bind to operator; transcription of structural genes
• presence of co-repressor activates repressor by altering tertiary structure and making it able to bind to operator; no transcription of structural genes

Question 8

Positive inducible control

Answer 8

• activator unable to bind to operator in normal conformation, preventing binding of RNA polymerase to promoter; no transcription of structural genes
• presence of inducer activates activator by altering tertiary structure and making it able to bind to operator; transcription of structural genes

Question 9

Positive repressible control

Answer 9

• activator normally bound to operator, allowing RNA polymerase to bind to promoter; transcription of structural genes
• presence of co-repressor inactivates activator by altering tertiary structure and making it unable to bind to operator; no transcription of structural genes

Question 10

What kind of operon is the lac operon?

Answer 10

Negative inducible operon

Question 11

Type of mechanism lac operon is associated with

Answer 11

Catabolism

Question 12

Structural genes in lac operon

Answer 12

• lac Z
• lac Y
• lac A

Question 13

What does lac Z encode for?

Answer 13

β-galactosidase

Question 14

What does lac Y encode for?

Answer 14

Permease

Question 15

What does lac A encode for?

Answer 15

Transacetylase

Question 16

Function of β-galactosidase

Answer 16

1. Hydrolyse lactose into glucose and galactose

2. Isomerise lactose into allolactose (inducer)

Question 17

Function of Permease

Answer 17

Allows efficient uptake of lactose into E. coli cells by active transport

Question 18

Function of Transacetylase

Answer 18

Transfer acetyl groups from acetyl CoA to β-galactosides to prevent build-up of toxic products formed by β-galactosidase

Question 19

Promoter of structural genes of lac operon

Answer 19

Lac P

Question 20

Operator of lac operon

Answer 20

Lac O

Question 21

Regulatory gene of lac operon

Answer 21

Lac I

Question 22

What does lac I encode for?

Answer 22

Active lac repressor

Question 23

(-ve) Regulation of lac operon - absence of lactose

Answer 23

• regulatory gene, lac I, constitutively transcribed to produce lac repressor
• active lac repressor binds to operator, preventing RNA polymerase from binding to promoter, lac P; no transcription of structural genes and lac operon is OFF

Question 24

(-ve) Regulation of lac operon - presence of lactose

Answer 24

• some lactose molecules enter E. coli cells aided by permease, before being converted to allolactose, an inducer, by β-galactosidase
• allocatose binds to repressor, inactivating it by altering tertiary structure and making it unable to bind to operator
• RNA polymerase binds to promoter; transcription of structural genes and lac operon is ON

Question 25

(+ve) Regulation of lac operon - presence of lactose and glucose

Answer 25

- metabolise glucose preferentially; glucose imported cell, inactivating permease and preventing uptake of lactose - genes involved in metabolising other sugars are repressed through catabolite repression - glucose inhibits adenylyl cyclase and thus reduces conversion of ATP into cyclic adenine monophosphate (cAMP); fewer cAMP-CAP complexes formed - inactive CAP unable to bind to CAP binding site; decreased binding efficiency of RNA polymerase and thus reduced transcription of structural genes

Question 26

(+ve) Regulation of lac operon - presence of lactose, absence of glucose

Answer 26

- no inhibition of adenylyl cyclase by glucose and thus conversion of ATP into cyclic adenine monophosphate (cAMP); cAMP-CAP complexes formed - active CAP binds to CAP binding site; increased binding efficiency of RNA polymerase and thus increased transcription of structural genes

Question 27

Regulatory mechanism ensuring lac operon is OFF in presence of glucose

Answer 27

1. Low affinity of RNA polymerase for promoter | 2. Glucose-sensitive regulatory mechanism; CAP and CAP binding site

Question 28

What kind of operon is the trp operon?

Answer 28

Negative repressible operon

Question 29

Type of mechanism trp operon is associated with

Answer 29

Anabolism

Question 30

Structural genes in trp operon

Answer 30

- trp E - trp D - trp C - trp B - trp A

Question 31

Regulatory gene of trp operon

Answer 31

Trp R

Question 32

What does trp R encode for?

Answer 32

Inactive trp repressor

Question 33

Regulation of trp operon - high levels of tryptophan

Answer 33

- tryptophan, co-repressor, binds to inactive trp repressor, activating it by altering tertiary structure - trp repressor binds to operator, preventing RNA polymerase from binding to promoter; no transcription of structural genes

Question 34

Regulation of trp operon - low levels of tryptophan

Answer 34

- inactive trp repressor unable to bind to operator, RNA polymerase binds to promoter; transcription of structural genes